1. Field of the Invention
The present invention pertains generally to apparatus for monitoring the radiation within the core of a nuclear reactor and, more particularly, to such apparatus that will not obstruct refueling of the reactor.
2. Related Art
In many state-of-the-art nuclear reactor systems in-core sensors are employed for measuring the radioactivity within the core at a number of axial elevations. These sensors are used to measure the radial and axial distribution of the power inside the reactor core. This power distribution measurement information is used to determine whether the reactor is operating within nuclear power distribution limits. The typical in-core sensor used to perform this function is a self-powered detector that produces an electric current that is proportional to the amount of fission occurring around it. This type of sensor does not require an outside source of electrical power to produce the current and is commonly referred to as a self-powered detector and is more fully described in U.S. Pat. No. 5,745,538, issued Apr. 20, 1998, and assigned to the Assignee of this invention. FIG. 1 provides a diagram of the mechanisms that produce the current I(t) in a self-powered detector element 10. A neutron sensitive material such as vanadium is employed for the emitter element 12 and emits electrons in response to neutron irradiation. Typically, the self-powered detectors are grouped within instrumentation thimble assemblies. A representative in-core instrumentation thimble assembly is shown in FIG. 2. The signal level generated by the essentially non-depleting neutron sensitive emitter element 12 shown in FIG. 1, is low, however, a single, full core length neutron sensitive emitter element provides an adequate signal without complex and expensive signal processors. The proportions of the full length signal generated by the single neutron sensitive emitter element attributable to various axial regions of the core are determined from apportioning the signal generated by different lengths of gamma sensitive elements 14 which define the axial regions of the core and are shown in FIG. 2. The apportioning signals are ratioed which eliminates much of the effects of the delayed gamma radiation due to fission products. The in-core instrumentation thimble assemblies also include a thermocouple 18 for measuring the temperature of the coolant exiting the fuel assemblies. The electrical signal output from the self-powered detector elements and the thermocouple in each in-core instrumentation thimble assembly in the reactor core are collected at the electrical connector 20 and sent to a location well away from the reactor for final processing and use in producing the measured core power distribution.
FIG. 3 shows an example of a core monitoring system presently offered for sale by Westinghouse Electric Company LLC with the product name WINCISE™ that employs fixed in-core instrumentation thimble assemblies 16 within the instrument thimbles of fuel assemblies within the core to measure the core's power distribution. Cabling 22 extends from the instrument thimble assemblies 16 through the containment seal table 24 to a signal processing cabinet 26 where the outputs are conditioned, digitized and multiplexed and transmitted through the containment walls 28 to a computer workstation 30 where they can be further processed and displayed. The thermocouple signals from the in-core instrumentation thimble assemblies are also sent to a reference junction unit 32 which transmits the signals to an inadequate core cooling monitor 34 which communicates with the plant computer 36 which is also connected to the workstation 30. Because of the hostile environment, the signal processing cabinet 26 has to be located a significant distance away from the core and the signal has to be sent from the detector 16 to the signal processing cabinet 26 through specially constructed cables that are extremely expensive and the long runs reduce the signal to noise ratio. Unfortunately, these long runs of cable have proved necessary because the electronics for signal processing has to be shielded from the highly radioactive environment surrounding the core region.
In previous nuclear plant designs, the in-core detectors entered the reactor vessel from the lower hemispherical end and entered the fuel assemblies instrumentation thimble from the bottom fuel assembly nozzle. In at least some of the current generation of nuclear plant designs, such as the AP1000 nuclear plant, the in-core monitoring access is located at the top of the reactor vessel, which means that during refueling all in-core monitoring cabling will need to be removed before accessing the fuel. A wireless in-core monitor that is self-contained within the fuel assemblies and wirelessly transmits the monitored signals to a location remote from the reactor vessel would allow immediate access to the fuel without the time-consuming and expensive process of disconnecting, withdrawing and storing the in-core monitoring cables before the fuel assemblies could be accessed, and restoring those connections after the refueling process is complete. A wireless alternative would thus save days in the critical path of a refueling outage. A wireless system also allows every fuel assembly to be monitored, which significantly increases the amount of core power distribution information that is available.
However, a wireless system requires that electronic components be located at or very near the reactor core where gamma and neutron radiation and high temperatures would render semiconductor electronics inoperable within a very short time. Vacuum tubes are known to be radiation insensitive, but their size and current demands have made their use impractical until recently. Recent developments in micro-electromechanical devices have allowed vacuum tubes to shrink to microscopic sizes and significantly reduced power draw demands.
Accordingly, it is an object of this invention to improve the critical path for refueling a reactor by significantly reducing the number of cables attached to the reactor head that would have to be removed and reconnected in the course of the refueling process.
It is a further object of this invention to provide a fuel assembly with a self-contained instrument thimble assembly that can be inserted into the core of a nuclear reactor and placed in operation without the necessity of routing cabling and connectors through the reactor vessel to activate the instrumentation.
It is an additional object of this invention to increase the amount of in-core power distribution data that is communicated to the plant operator.